Fix for plugin installation via web interface (#5646)

* Fix for plugin installation via web interface

- Previous implementation did not correctly catch error output
- Refactored into new file plugin.installer.py
- Provides better "output" when plugin does install correctly

* Remove unnecessary debug output

* Update unit test

* Remove placeholder code

InvenTree/templates/js/translated/plugin.js

@@ -157,6 +157,9 @@ function installPlugin() {
         onSuccess: function(data) {
             let msg = '{% trans "The Plugin was installed" %}';
             showMessage(msg, {style: 'success', details: data.result, timeout: 30000});
+
+            // Reload the plugin table
+            $('#table-plugins').bootstrapTable('refresh');
         }
     });
 }